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Swimming Pool and Spa Evaporation: How Much Water Are You Losing?
- by Mathieu Aubertin
Pool and Spa Evaporation: How Much Water Are You Losing?
You've noticed that the water level in your pool or spa has dropped since last week, and one question immediately comes to mind: normal evaporation, or a leak? The good news is that in the vast majority of cases, it's simply evaporation—a perfectly normal phenomenon, but one that varies enormously from one installation to another. Here's what you need to understand to tell the difference, and how to get an estimate that truly reflects your pool or spa.
What exactly is evaporation?
Evaporation is the process by which water changes from a liquid to a gaseous state at the surface of your pool. This phenomenon occurs continuously, day and night, as soon as the ambient air is not already saturated with humidity. The greater the difference between the water temperature and the air temperature, and the drier the air, the faster the evaporation.
This is why a heated pool on a cool autumn evening can visibly evaporate faster than in the middle of summer—even if the air seems "less warm."
Why your pool loses more (or less) water than your neighbor's
Two pools of the same size, in the same city, can have very different evaporation rates. Several factors come into play:
- Wind ➡️ a completely exposed pool loses much more water than a sheltered pool between two houses, a fence, or a hedge.
- Sunlight ➡️ full sun heats the water and accelerates evaporation; the shade of a tree or building slows it down.
- Heating ➡️ water kept at a consistent temperature (heated pool, spa) evaporates faster than water at natural temperature.
- A cover or lid ➡️ covering the pool when not in use significantly reduces evaporation.
- Humidity and current weather ➡️ dry and windy air accelerates everything; humid and calm air slows everything down.
It's the combination of all these elements—not just one of them—that determines how much water your pool actually loses.
How much water loss is normal?
There is no single valid figure for all pools. A rule of thumb often cited in the industry puts average evaporation at around 5 to 8 mm per day under typical conditions—but this figure can easily double or triple on a hot, dry, and windy day, or conversely be much lower in humid and calm weather, especially if the pool is covered.
In other words: a generic number doesn't tell you much about your pool. This is precisely why we have developed a tool that calculates your evaporation based on the actual weather in your city rather than a general average—more details below.
Evaporation or leak: how to decide
The most reliable way to determine if your water loss is normal is the bucket test:
- Fill a bucket with water and place it on a pool step, so that the water level inside is equal to that of the pool.
- Mark both levels—the one in the bucket and the one in the pool.
- Wait 24 hours, ideally on a day without rain.
- Compare the two drops: the bucket has no swimmers, no filtration system, only evaporation.
👉 If your pool has dropped significantly more than the bucket, a leak is possible and warrants inspection. If both drops are similar, your water is simply evaporating—nothing to worry about.
Reducing evaporation: what really works
You cannot eliminate evaporation, but you can significantly reduce it:
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Cover the pool when not in use—by far the most effective measure, especially for a spa.
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Avoid unnecessary overheating of the water beyond what you truly need.
- Block the wind around the pool (hedge, fence, screen) if the exposure is very open.
A solar cover, in addition to limiting evaporation, helps retain water heat—which directly translates into water and energy savings over an entire season.
Get your own estimate with the Barracuda calculator
Stop guessing—calculate.
Our evaporation calculator uses the actual hourly weather in your city, as well as the specific characteristics of your pool or spa (wind exposure, sunlight, heating, cover) to give you an estimate that reflects your situation—not a generic average. In a few minutes, you'll know if your water loss is normal, or if it needs to be monitored.
Conclusion
A drop in water level doesn't automatically mean a leak: it's most often evaporation, a normal phenomenon whose extent depends on your local weather, your pool's exposure, and your usage habits. The bucket test remains the simplest way to confirm that everything is normal, and our evaporation calculator gives you an estimate based on your real situation in a few clicks.
Frequently Asked Questions about Evaporation
How long does it take to notice normal evaporation?
Under average conditions, a drop of a few millimeters per day is usually invisible to the naked eye for several days. If you notice a significant drop within 24 to 48 hours, the bucket test can quickly confirm if it's normal.
Does a solar cover completely eliminate evaporation?
No, but it significantly reduces it when used systematically when the pool is not in use. A pool or spa that is never covered will continue to evaporate normally, regardless of the quality of the cover.
Why does my spa seem to lose more water, proportionally, than my pool?
A spa is almost always heated to a higher temperature than a pool, which accelerates evaporation per unit of surface area. Since its surface is small, every millimeter lost also appears more visible in the sight glass or level indicator.
